For more than half a century, the cathode ray tube (CRT) has been the principal device for displaying visual information. Although CRTs have been endowed during that period with remarkable display characteristics in the areas of color, brightness, contrast and resolution, they have remained relatively bulky and power hungry. The advent of portable computers has created intense demand for displays which are lightweight, compact, and power efficient. Although liquid crystal displays are now used almost universally for laptop computers, contrast is poor in comparison to CRTs, only a limited range of viewing angles is possible, and in color versions, they consume power at rates which are incompatible with extended battery operation. In addition, color screens tend to be far more costly than CRTs of equal screen size.
As a result of the drawbacks of liquid crystal display technology, thin film field emission display technology has been receiving increasing attention by industry. Flat panel display utilizing such technology employ a matrix-addressable array of pointed, thin-film, cold field emission cathodes in combination with a phosphor-luminescent screen. Somewhat analogous to a cathode ray tube, individual field emission structures are sometimes referred to as vacuum microelectronic triodes. The triode elements are a cathode (emitter tip), a grid (also referred to as the gate), and an anode (typically, the phosphor-coated element to which emitted electrons are directed).
Although the phenomenon of field emission was discovered in the 1950's, extensive research by many individuals, such as Charles A. Spindt of SRI International, improved the technology to the extent that prospects for its use in the manufacture of inexpensive, low-power, high-resolution, high-contrast, full-color flat displays appeared promising. Realizing that field emission technology was no longer a technology that should be relegated to the care of scientists interested primarily in pure research, a dozen or so companies joined the race to develop a practical flat panel field emission display.
Conventional field emission displays are constructed such that a column signal activates a single conductive strip within the grid, while a row signal activates a conductive strip within the emitter base electrode. At the intersection of an activated column and an activated row, a grid-to-emitter voltage differential sufficient to induce field emission will exist, causing illumination of an associated phosphor on the phosphorescent screen. There are a number of problems associated with this conventional matrix-addressable field-emission display architecture. In order for field emission to occur, the voltage differential between a row conductor and a column conductor must be at least equal to a voltage which will provide acceptable field emission levels. Field emission intensity is highly dependent on several factors, the most important of which is the sharpness of the cathode emitter tip and the intensity of the electric field at the tip. Although a level of field emission suitable for the operation of flat panel displays has been achieved with emitter-to-grid voltages as low as 60 volts (and this figure is expected to decrease in the coming years due to improvements in emitter structure design and fabrication) emission voltages will probably remain far greater than 5 volts, which is the standard CMOS, NMOS, and TTL "1" level. Thus, if the field emission threshold voltage is at 60 volts, row and column lines will, most probably, be designed to switch between 0 and either +30 or -30 volts in order to provide an intersection voltage differential of 60 volts. Hence, it will be necessary to perform high-voltage switching as these row and column lines are activated. Not only is there a problem of building drivers to switch such high voltages, but there is also the problem of unnecessary power consumption because of the capacitive coupling of row and column lines. That is to say, the higher the voltage on these lines, the greater the power required to drive the display.
In addition to the problem of high-voltage switching, conventional field emission displays are also prone to low yield and low reliability due to the possibility of emitter-to-grid shorts. Such a short affects the voltage differential between the emitters and grid within the entire array, and may well render the entire array useless, either by consuming so much power that the supply is not able to maintain a voltage differential sufficient to induce field emission, or by actually generating so much heat that a portion of the array is actually destroyed.
A field emission display architecture, which is the subject of U.S. Pat. No. 5,210,472, overcomes the problems of high-voltage switching and emitter-to-grid shorts, which, in turn, ameliorates the problem of display power consumption. The new architecture (hereinafter referred to as the "dual series-coupled transistor, low-voltage-switching field emission display architecture") permits the switching of a high pixel activation voltage with low signal voltages that are compatible with standard CMOS, NMOS, or other integrated circuit logic levels. Instead of having rows and columns tied directly to the cathode array, they are used to gate at least one pair of series-connected field effect transistors (FETs), each pair when conductive coupling the base electrode of a single emitter node to a potential that is sufficiently low, with respect to a higher potential applied to the grid, to induce field emission. Each row-column intersection (i.e. pixel) within the display may contain multiple emitter nodes in order to improve manufacturing yield and product reliability. In a preferred embodiment, the grid of the array is held at a constant potential (V.sub.FE), which is consistent with reliable field emission when the emitters are at ground potential. A multiplicity of emitter nodes are employed, one or more of which correspond to a single pixel (i.e., row and column intersection). Each emitter node has its own base electrode, which is groundable through its own pair of series-coupled field-effect transistors by applying a signal voltage to both the row and column lines associated with that emitter node. One of the series-connected FETs is gated by a signal on the row line; the other FET is gated by a signal on the column line. Also in the preferred embodiment of the invention, each emitter node contains multiple cathode emitters. Hence, each row-column intersection controls multiple pairs of series-coupled FETs, and each pair controls a single emitter node (pixel) containing multiple emitters.
The regulation of cathode-to-grid current has become a major issue in the design of field emission displays, as the issues of cathode life expectancy, low power consumption, and stability requirements are addressed.
The issue of current regulation has been addressed with respect to conventionally constructed flat panel field emission displays, such as the one depicted in FIG. 1. For example, in U.S. Pat. No. 4,940,916, Michel Borel and three colleagues disclose a field emission display having a resistive layer between each cathode (emitter tip) and an underlying conductive layer. In a subsequent U.S. Pat. No. 5,162,704, Yoichi Bobori and Mitsuru Tanaka disclose a field emission display having a diode in series with each emitter tip.
In U.S. patent application Ser. No. 08/011,927, now issued as U.S. Pat. No. 5,357,172, a method is disclosed for reducing power consumption and enhancing reliability and stability in the low-voltage switching field emission display architecture by regulating cathode emission current. This is achieved by placing a resistor in series with each pair of series-coupled low-voltage switching MOSFETs. As heretofore explained, each MOSFET pair couples an emitter node, which contains one or more field emitter tips, to ground. The resistor is coupled directly to the ground bus and to the source of the MOSFET furthest from the emitter node. By coupling the current-regulating resistor directly to the ground bus, stable current values independent of cathode voltage are achieved over a wide range of cathode voltages.
A functional, monochrome, 1.75 cm-diagonal prototype of the dual, series-coupled low-voltage switching field emission display architecture, which incorporated the current-regulating resistors of U.S. Pat. No. 5,357,172 in the emitter grounding circuits, was constructed in 1993 by Micron Display Technology, Inc. of Boise, Id. FIG. 1 is representative of the pixel control circuitry for a single emitter node of the monochrome prototype display. Ideally, each pixel within the display will have multiple emitter nodes so that if one node is defective, the pixel will still function. The circuitry is characterized by a conductive grid 11, which is maintained at a constant potential, V.sub.GRID, a transparent screen 12, and a phosphorescent layer 13, which coats the screen. The grid 11, the screen 12, and the phosphorescent layer 13 are continuous throughout the entire display. The node is depicted as having only two field emission cathodes 14A and 14B (also referred to as emitter tips). In actuality, a larger number of cathodes is desirable, as illumination uniformity in the display is thereby enhanced. Each of the emitters 14A and 14B is connected to a base electrode 15 that is common to only the emitters of the emitter node. In order to induce field emission, base electrode 15 is grounded through a pair of series-coupled field-effect transistors Q1 and Q2 and current-regulating resistor R1. Resistor R1 is interposed between the source of transistor Q1 and ground. Transistor Q1 is gated by a row line RL, while transistor Q2 is gated by a column line CL. It should be noted that a functionally equivalent circuit is created if column line CL controls the gate of transistor Q1 and row line RL controls the gate of transistor Q2. Standard logic signal voltages for CMOS, NMOS, TTL and other integrated circuits are generally 5 volts or less, and may be used for both column and row line signals. A pixel is turned off (i.e., placed in a non-emitting state) by turning off either or both of the series-connected FETs (Q1 and Q2). From the moment that at least one of the FETs becomes non-conductive (i.e., the gate-to-source voltage V.sub.GS drops below the device threshold voltage V.sub.T, electrons will continue to be discharged from the emitter tips corresponding to that pixel until the voltage differential between the base and the grid is just below emission threshold voltage. In order to improve yield and to minimize array power consumption, an optional fusible link FL is placed in series with the pull-down current path from base electrode 153 to ground via transistors Q1 and Q2. Fusible link FL may be blown during testing if a base-to-emitter short exists within that emitter group, thus isolating the shorted group from the rest of the array.
Although performance of the prototype display exceeded expectations in many respects, it was noted that, under certain operating conditions, unintended pixel emission occurred when the transistor nearest ground was turned "off" and the transistor nearest the emitter tip was turned "on". This phenomenon resulted in a low-intensity background glow over which desired images were superimposed. This problem is believed to be associated with the parasitic capacitance of the node between each pair of transistors in the pixel grounding path (hereinafter the intermediate node). The following sequence of events is the most likely cause of the phenomenon. The transistor nearest the emitter node is turned "off" by a low logic signal on its gate. Then, the transistor nearest ground is turned "off" by a low logic signal on its gate, resulting in the intermediate node being at ground potential. When the transistor nearest the emitter is then turned "on" by a high logic signal on its gate, the difference in potential between the emitter node and the grid is sufficient to cause field emission until the intermediate node has emitted a number of electrons sufficient to cause the difference in potential between the emitter node and the grid to drop below the emission threshold.
In 1994, Micron Display Technology, Inc. constructed a functional, color, 1.75 cm-diagonal prototype employing an improved two-transistor pixel control circuit that remedied the heretofore described unintended pixel emission phenomenon. The improved pixel control circuit, which is depicted in FIG. 2, places only a single transistor (the primary control transistor) QP in the grounding path. The problematic intermediate node is thus eliminated from the grounding path. The gate of transistor QP is controlled by a row line RL, which passes through a secondary pixel control transistor QS. Transistor QS, in turn, is controlled by a column line CL. Thus, only when both the signals on both row line RL and column line CL are high is the primary pixel control transistor QP in an "on" state. It should be noted that a functionally equivalent circuit is created if column line CL controls the gate of transistor QP and row line RL controls the gate of transistor QS. Capacitor C1, which is charged when signals on both row line RL and column line CL are high, retains pixel information between raster scans. The improved circuit, like the original dual, series-coupled transistor pixel control circuit, requires two transistors at the intersection of each row and column line in the display, and also requires the routing of three signal lines through the display array (i.e., row, column and ground).